A CMOS image sensor is a semiconductor device embedded with an integrated circuit which generally has several hundreds of thousands to several millions of pixels to convert optical energy into an electrical signal such that original images of persons or matters are converted into electrical signals in the image sensor.
FIG. 1 is a view showing the structure of an image sensor according to the related art.
Referring to FIG. 1, a semiconductor substrate 11 is formed therein with a photodiode PD, and formed thereon with a multi-layer metal line structure including a plurality of interlayer dielectric layers ILD1, IMD1, IMD2, IMD3, and ILD2 and metal lines M1, M2, M3, and M4 within the interlayer dielectric layers.
A color filter layer 20 is formed on the uppermost interlayer dielectric layer ILD2 to correspond to the PD, and a microlens 30 is formed on the color filter layer 20 to also correspond to the PD.
Etch stop layers 12 are typically formed in layers between the metal lines and the interlayer dielectric layers for use in performing a dual damascene process for forming the metal lines and vias connecting the metal lines.
For example, when the metal line includes copper, a via and the metal line is formed through the dual damascene process. Copper interconnections have become more widely used as compared to aluminum interconnections because the thickness of the copper interconnection can be more easily reduced as compared with that of an aluminum interconnection. Therefore, as image sensors become more highly integrated, but continue to include a high number of metal lines and interlayer dielectric layers, the copper interconnection is advantageous when a microlens is formed to improve the distance between the microlens and the photodiode.
In addition, because an etch stop layer, which is used in forming a copper line through a dual damascene process, may include silicon nitride (Si3N4) or silicon carbide (SiC) having a transmittance characteristic inferior to that of silicon oxide (SiO2) used for an insulating layer, a light characteristic may become degraded.
In other words, silicon nitride and silicon carbide have refractive indices of about 2.1 and about 2.4, respectively. Accordingly, if an etch stop layer having a high refractive index is formed between insulting layers that are formed of silicon oxide having a refractive index of about 1.5, light is reflected by the number of metal line layers (due to the number of etch stop layers), so that light loss is increased.